Electrochemical double-layer capacitor using organosilicon electrolytes

ABSTRACT

Disclosed are supercapacitors having organosilicon electrolytes, high surface area/porous electrodes, and optionally organosilicon separators. Electrodes are formed from high surface area material (such as porous carbon nanotubes or carbon nanofibers), which has been impregnated with the electrolyte. These type devices appear particularly suitable for use in electric and hybrid electric vehicles.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

This invention was made with United States government support awarded bythe following agency: NSF 0210806. The United States government hascertain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to electric double-layer capacitors(EDLCs), which are sometimes also referred to in the art as“electrochemical capacitors”, “supercapacitors”, or “ultracapacitors”.More particularly, it relates to the use of organosilicon electrolytes,and in some instances optionally an organosilicon separator and/orbinder in such devices.

A schematic example of a known type of EDLC is depicted in FIG. 1. Thisdrawing shows an electrochemical double-layer capacitor 10 having twoelectrodes 11 which are kept from electrical contact by a separator 12.There are current collectors 13 at opposite ends of the device. Theelectrode consists of a porous material 14 and an electrolyte 15. Boththe separator 12 and the porous material 14 are immersed in theelectrolyte 15.

The electrolyte allows ions to move freely through the separator. Theseparator is designed to prevent electrical contact between theelectrodes which otherwise might create a short circuit in the device.

The current collecting plates 13 are in contact with the backs of theelectrodes 11. Electrostatic energy is stored in polarized liquidlayers, which form when a potential is applied across two of theelectrodes. A “double layer” of positive and negative charges is formedat the electrode-electrolyte interface.

Electrochemical double-layer capacitors provide energy storage as wellas pulse power delivery. This is useful in many applications, andparticularly where high power pulses are desired. In this regard, otherenergy storage devices such as batteries or fuel cells, which can storelarge amount of energy, cannot deliver high power pulses. Anotheradvantage of supercapacitors is that they can be charged rapidly.

To be optimally effective such devices must, among other properties,have low internal resistance, store large amounts of charge, bephysically strong, be stable at desired voltages, and be otherwisecompatible with the usage environment. Therefore, there are many designparameters that must be considered in construction of such devices.

Electrochemical double-layer charge storage is a surface process and thesurface characteristics of the electrode material can greatly influencethe capacitance of the device. In this regard, the electrodes of asupercapacitor are typically made of very high surface area materials(e.g. porous carbon or carbon aerogels, carbon nanotubes, carbon foamsor fibers, porous metal oxides) in order to maximize the surface area ofthe double-layers. High electrode-electrolyte interfacial surface areaand nanometer dimensions of the charge separation layer result in highspecific capacitance of electrodes so that high energy densities can beachieved in EDLCs.

It is particularly desirable that electrode materials in EDLCs havelarge pore diameters and good pore connectivity, so that electrolyte caneasily penetrate the pores, facilitating rapid ion motion and highconductivity. Electrons can then easily flow from the electrode to thecurrent collector and vice versa.

Carbon is the most widely used high surface area electrode material forEDLCs. Carbon can form specific textures, providing high surface area.High surface area carbon is particularly desirable because it can formmesopores or graphite crystallites suitable for ions intercalation.

See also U.S. Pat. Nos. 5,963,417 and 6,721,168, as well as U.S. patentapplication publication 2003/0030963 regarding a variety of other knownhigh surface area materials for electrodes for various applications. Thedisclosure of these patents and patent application publications, and ofall other publications referred to herein, is incorporated by referenceas if fully set forth herein.

Aqueous and some organic electrolyte solutions have in the past beenused in electrochemical double layer capacitors. Aqueous electrolytes,as compared to earlier organic electrolytes, provide lower equivalentseries resistance improving the time constant of a supercapacitor andproviding high power densities. However, they were not stable at theoperating voltages exceeding the electrolysis voltage of water (1.23 V)and that made organic liquid electrolytes preferable to aqueouselectrolyte solutions for many commercial applications.

Organic liquid electrolytes that can be used in supercapacitors shouldpreferably have higher ionic conductivity. As an example, acetonitrileproviding high ionic conductivity has been used in such electrolytes.However, acetonitrile is a hazardous flammable and toxic material, whichproduces highly toxic products (HCN and CO) upon combustion and thermaldecomposition.

Other previously used organic liquid electrolytes, like those based onalkyl carbonates (ethylene carbonate, propylene carbonate, andy-butyro-lactone, or dimethylcarbonate, diethylcarbonate, andethylmethylcarbonate, for example) are highly flammable. They have lowerionic conductivity as compared to aqueous electrolytes or electrolytesbased on acetonitrile, and this causes higher internal losses of storedenergy and power density of the supercapacitor.

In unrelated work ion-conducting organosilicon polymer electrolytes havebeen proposed for lithium-polymer battery applications. See e.g. U.S.Pat. No. 6,337,383 and U.S. patent application publications2003/0198869, 2004/0197665, 2004/0214090 and 2004/0248014. However,there was no teaching in these references to combine an organosiliconmaterial with a high surface area material to create a supercapacitorelectrode, or any suggestion to use ion-conducting organosiliconmaterial as a binder or as a separator layer in supercapacitors.

Hence, there is a continuing need for EDLCs with improved safety andother characteristics, particularly those capable of operating stably athigher voltages.

SUMMARY OF THE INVENTION

In one aspect the invention relates to electrochemical double-layercapacitors based on organosilicon electrolytes and/or an organosiliconseparator. In one embodiment there is an electrochemical double-layercapacitor which has a first electrode having a first porous materialmixed with a first organosilicon electrolyte, and a second electrode.There may also be a separator disposed between the first and secondelectrodes.

In the claims, the term “porous” is being used not only in itsconventional sense, but also to include fiber arrays, nanostructuredmaterials such as carbon nanofiber arrays, nanostructured graphite, andother materials in which the structure provides spacings creating a highsurface area accessible for ions and electrolyte molecules. Thus, a“porous” material will include without limitation, porous carbon, carbonwith crystallites suitable for ions intercalation, porous metal oxides,nanostructured metal oxides, etc.

Preferably the first organosilicon electrolyte is a solution of alithium salt in an ion-conducting organosilicon compound, such as anoligoethyleneoxide-substituted organosilane (with one or moreethyleneoxide substituents) see FIG. 4. It is also highly preferred thatthe second electrode be a second porous material which is the same ordifferent from the first porous material and is mixed with a secondorganosilicon electrolyte which is the same or different from the firstorganosilicon electrolyte.

The first and second porous materials are preferably selected from thegroup consisting of porous carbon, non-porous carbon with crystallitessuitable for ions intercalation, porous metal oxides, or carbon modifiedwith an electroactive polymer like polypyrrole or polyaniline. The firstporous material is the same or different from the second porousmaterial. For example, both could be carbon nanofibers or carbonnanotubes.

Where carbon nanofibers form the porous material they can optionally bein the form of a fiber “forest” in which the “trees of the forest” aresufficiently separated so that a row of the fibers can be the porousmaterial for the first electrode, a second row of the fibers can be theporous material for the second electrode, a single pool of organosiliconion conducting material mixed with a lithium salt can surround bothelectrodes, and no additional separator is required.

Also, the separator can be made of a solid organosilicon material (e.g.a cross-linked polysiloxane having oxyethylene moieties), and there canbe a first current collector adjacent the first electrode and a secondcurrent collector adjacent the second electrode, with both collectorspreferably made from a highly conductive material, for example metalsuch as aluminum, nickel, copper, titanium, molybdenum, silver, gold orsteel.

In another aspect the invention provides an electrode for use in anelectrochemical capacitor. The electrode has a porous material such asthose noted above, and an organosilicon electrolyte. Again, thepreferred organosilicon electrolyte is a mixture of oligoethyleneoxidesubstituted silane or siloxane with a lithium salt, e.g.lithium(bis-oxalato)borate.

In yet another aspect the invention provides an electrochemicaldouble-layer capacitor comprising a separator disposed between first andsecond electrodes, wherein conventional separator film (e.g. “Celgard”)can be used or optionally the separator comprises an organosiliconmaterial, such as a cross-linked oligoethyleneoxide substitutedorganosiloxane.

Ion-conducting organosilicon material is mixed with a lithium salt toprepare the electrolyte, and the resulting electrolyte is added to aporous material to form a supercapacitor electrode having particularlydesirable properties. Also, optionally, ion-conducting solidorganosilicon materials can be used as a binder of the porous materialfor structural stability. Also, optionally, ion-conducting solidorganosilicon materials can be used to form a separator in suchsupercapacitors.

Preferred electrolytes of the present invention have high roomtemperature ionic conductivity (hence providing lower equivalent seriesresistance), high thermal and electrochemical stability (thereforehigher operating voltages and temperatures), and lower volatility,toxicity and flammability which in turn provides safety and performanceenhancement. In addition, they are much less hydrophilic than certainother conventional electrolytes and therefore are more compatible withhydrophobic carbonaceous electrode materials and dielectric hydrophobicconventional polymer separators based on polyethylene (PE) orpolypropylene (PP) or poly-tetrafluoroethylene (PTFE).

Particularly advantageous is that the present invention, in someembodiments, permits operation at high voltages see FIG. 9. Thus, ourEDLCs, as compared to conventional supercapacitors, allow obtainingdevices with high energy density: E=½ CV², where E is energy stored insupercapacitor in Wh per kg, C is capacitance in Farads, V is electricpotential in Volts; and power density: P=V²/4R, where P is a powerdensity of the supercapacitor in W per kg, V is applied potential inVolts and R is equivalent series resistance in ohms.

Also, our preferred electrolytes, and thus our preferred electrodes, arerelatively easy to make, and can be synthesized in high yields fromavailable and relatively inexpensive starting materials. They are alsostable in storage and processing.

As some of our preferred electrolytes are solids with high roomtemperature ionic conductivity, the EDLC can be fabricated as anall-solid-state device. Such an all-solid-state supercapacitor can besafely packed in an ultra thin laminated foil material. The laminationoperation is simple and inexpensive. Also, a rolled-sheet manufacturingprocess can be used to fabricate components of the solid organosiliconsupercapacitor. This technology can be used in exceptionally costeffective, high speed and high volume industrial supercapacitorproduction.

The all-solid-state organosilicon supercapacitors do not need to bepacked in rigid, hermetically sealed metal-containers, which in turnwould require complicated and cost consuming winding, canning andhermetic sealing operations. Hermetically sealed metal containers reduceeffective volume, especially in large packs, resulting in increase ofthe weight and volume proportion of the housing metal material in liquidmulticell devices, in comparison to the all-solid-state supercapacitors.

The above and still other advantages of the present invention will beapparent from the description that follows. It should be appreciatedthat the following description is merely of the preferred embodiments ofour invention. The claims should therefore be looked to in order tounderstand the full claimed scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts, in schematic form, a known structure forsupercapacitors;

FIG. 2 illustrates a synthesis of a precursor for a preferredcross-linked solid organosilicon electrolyte;

FIG. 3 illustrates the cross-linking of the precursor to create apreferred cross-linked solid organosilicon electrolyte useful inconnection with the present invention;

FIG. 4 illustrates a synthesis of a liquid silane electrolyte;

FIG. 5 illustrates a synthesis of a liquid disiloxane electrolyte;

FIG. 6 illustrates a synthesis of a polysiloxane electrolyte;

FIG. 7 is a schematic depiction of a two-electrode Teflon Swageloksupercapacitor cell, in which electrodes and a separator of the presentinvention have been positioned;

FIG. 8 illustrates the results of cyclic voltammetry test for an EDLCdevice of the present invention using arrayed nanofiber electrodes,conventional separator film and liquid organosilicon electrolyte;

FIG. 9 illustrates the results of galvanostatic charge/dischargestability tests for an EDLC device of the present invention usingarrayed nanofiber electrodes, conventional separator film and liquidorganosilicon electrolyte;

FIG. 10 illustrates a structure of a monolithic EDLC device withelectrodes fabricated from carbon nanofiber patterned arrays grown onthe same substrate; and

FIG. 11 are surface electron microscopy photographs of monolithic EDLCdevice electrodes made of aligned arrays of carbon nanofibers useful inconnection with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Beginning with the prior art device of FIG. 1, to create embodiments ofthe present invention one replaces conventional liquid electrolyte 15with an organosilicon electrolyte in creating both electrodes 11.Optionally the cross-linked ion-conducting polymer can be used as abinder to impregnate high surface area porous electrode material toensure cohesion of the active material. Also optionally thenon-conductive conventional porous separator 12 can be replaced with thesame dimension separator, albeit formed of solid ion-conductingpolysiloxane material.

A. The Porous Electrode Material

Our preferred electrodes have as one component porous solid materials,particularly porous or nanostructured non-porous carbon or carbon aerogels, carbon nanotubes, carbon foams or carbon nanofibers. Porous andnanostructured metal oxides are also acceptable materials.

Carbon materials are particularly preferred because they can formspecific porous textures, providing high surface area. Formation ofmesopores or graphite crystallites suitable for ions intercalation incarbon materials is an important factor in determining the quality ofcarbon as an electrode material. See generally E. Frackowiak et al.,Carbon materials for the electrochemical storage of energy incapacitors, 39 Carbon 937-950 (2001); G. Gryglewicz et al., Effect ofpore size distribution of coal-based activated carbons on double layercapacitance, 50 Electrochimica Acta 1197-1206 (2005); and U.S. Pat. No.6,721,168.

Such porous materials may have relatively large pore diameters and goodpore connectivity, so that electrolyte can easily penetrate the pores,facilitating rapid ion motion and high conductivity. Electrons can theneasily flow from the electrode to the current collector and vice versa.While a variety of porous structures are suitable, pores with widths inthe range of 2 nm to 50 nm appear particularly desirable.

Alternatively, high surface area nanostructured graphite materialsforming crystallites suitable for ions intercalation can be used in theembodiments of this invention (see generally U.S. Pat. No. 6,721,168).

Carbon nanotubes and carbon nanofibers also be used as porous electrodematerials. For information on previous use of carbon nanotubes as anelectrode material in supercapacitors see generally: Y. Hee et al.,Carbon Nanotube-Based Supercapacitors, 1 Encyclopedia Of Nanoscience AndNanotechnology 625-634, American Scientific Publishers (2004); M.Hughes, Carbon Nanotube-Conducting Polymer Composites InSupercapacitors, 1 Encyclopedia Of Nanoscience And Nanotechnology447-459, American Scientific Publishers (2004); E. Frackowiak, CarbonNanotubes For Storage Of Energy: Supercapacitors, Encyclopedia ofNanoscience and Nanotechnology, Marcel Dekker, New York (2004).

Carbon nanotubes having an interconnected network of molecules form openmesopores, facilitating fast ion transport due to the high accessibilityof the electrode-electrolyte interface. High electronic percolationconductivity of carbon nanotubes is another important property, whichcomplements this particular application.

Pristine carbon nanotube molecules constitute concentrically rolledgraphite sheets capped by six pentagons at each end. The end caps do notallow electrolyte molecules and ions to penetrate the internal volume ofthe tubes. Also, due to the high aggregation forces carbon nanotubesusually form strong bundles in the bulk. Capping and aggregation ofcarbon nanotube molecules in bundles significantly reduces theireffective charge separation surface, and thus their utility as asupercapacitor electrode material.

To increase active surface area and volume of mesopores in carbonnanotubes and to disperse their bundles, the pristine material has to beactivated. Activation of the carbon nanotubes can be achieved bychemical treatment with strong oxidants, for example concentrated nitricacid. See generally S. Tsang et al., A simple chemical method of openingand filling carbon nanotubes, 372 Nature 159-162 (1994); Z. Jia et al.,Production of short multi-walled carbon nanotubes, 37 Carbon 903-906(1999).

Presence of the five-member rings in the hexagonal array at thenanotubes tips results in strain in the vicinity of the molecules andmakes them susceptible to oxidation of the tips on the double bondsconnecting the pentagons. Also treatment of the carbon nanotubes withnitric acid results in an increase of packing density and localalignment of molecules in the films favorable for this particularapplication. See generally C. Du et al., High power densitysupercapacitors using locally aligned carbon nanotube electrodes, 16Nanotechnology 350-353 (2005); C. Du et al., Carbon nanotube thin filmswith ordered structures, 15 J. Mat. Chem. 548-550 (2005).

Another approach to increase packing density and surface accessibilityis to fabricate vertically aligned arrays of nanotubes and nanofibers onelectroconductive bases. See generally Z. F. Ren et al., Synthesis ofLarge Arrays of Well-Aligned Carbon Nanotubes on Glass, 282 Science1105-1107 (1998); J. Li et al., Nanoelectronics: Growing Y-junctioncarbon nanotubes, 402 Nature, 253-254 (2000); and R. H. Baughman et al.,Carbon Nanotubes—the Route Toward Applications, 297 Science, 787-792(2002); S. Baker et al., Covalently Bonded Adducts of DeoxyribonucleicAcid (DNA) Oligonucleotides with Single-Wall Carbon Nanotubes: Synthesisand Hybridization, 2 Nano Letters, 1413-1417 (2002); C. S. Lee et al.,Electrically Addressable Biomolecular Functionalization of CarbonNanotube and Carbon Nanofiber Electrodes, 4 Nano Letters, 1713-1716(2004); U.S. patent application publication 2004/0235016.

These arrays can be fabricated using the well-known chemical vapordeposition technique. Carbon nanotube ordered arrays make it possible toobtain structures with controlled distances between nanotubes, avoidingcarbon nanotube bundles formation and increasing the available surfacearea of nanotubes. For example, in a recent publication fabrication andelectrochemical performance of structurally aligned multiwall carbonnanotube arrays as supercapacitor electrode materials was reported. Q.Chen et al., Fabrication and electrochemical properties of carbonnanotube array electrode for supercapacitors, 49 Electrochimica Acta4157-4161 (2004).

Aligned arrays of carbon nanotubes and nanofibers possess much betterelectrical contact with current collectors, which is important for theirapplication in electrode materials. In the pristine carbon nanotubes therandom network of nanotubes provides poor electrical contact becauseeach nanotube makes only a small number of point contacts to tubesunderneath it. This increases the resistance and decreases the abilityof the supercapacitor to respond to rapid changes in current loads.

In the vertically oriented arrays each nanotube or nanofiber moleculehas direct electrical contact with the underlying base, which serves asa current collector. Furthermore, the more open structure of thevertically-oriented carbon nanotubes and nanofibers leads to increasedaccessibility of ions, reducing the tortuous path that is characteristicof most other porous materials. The improved electrical contact andincreased accessibility of ions to the nanofiber surface both increasethe high-frequency response of the capacitors (i.e., the ability handlefast changes in current).

For applications such as electrical energy storage in automobiles, thisis important because operations such as acceleration, for example,require high instantaneous electrical current. We have demonstratedexperimentally that arrayed carbon nanofiber electrodes have anelectrical response that is flatter (i.e., less dependent on frequency)than for other electrodes such as carbon nanotubes, and that the carbonnanofiber electrodes have better high-frequency response than mats ofcarbon nanotubes.

Carbon nanofibers are also amenable to the fabrication of capacitorstructures in which both electrodes are fabricated on a single base thatprovides mechanical support and electrical contact. The patterned growthis possible because the nanofibers are grown by a catalyzed process, andby controlling the location of the catalyst it is possible to preciselypattern the spatial location of the nanofibers, and thus electrodes.This ability makes it possible to produce multiple capacitors on a smallstructure using well-known procedures such as optical lithography and/orelectron-beam lithography. This process eliminates the need for aseparator.

Both electrodes of a supercapacitor structure can be simultaneouslyfabricated side-by-side using array of carbon nanofibers to provide highsurface area. Such application may require using microelectronicsfabrication processes such as “spin-on” methods. Such monolithicstructures would also eliminate the need for a “separator” in an EDLC,potentially simplifying the overall construction of capacitor devices.

B. Electrolytes

To be useful as components of an electrolyte our organosilicon materialsmust be ion-conducting. Particularly preferred materials are theorganosilanes and organosiloxanes, such as those havingoligoethyleneoxide moieties (e.g. between 2 and 500 repeating units).These materials can have linear, branched, hyper-branched orcross-linked structure, and can be liquid (of varied viscosity), gel orsolid.

There may be just one silicon atom in the material (as is the case in(CH₃)₃SiO—(CH₂CH₂O)₃—CH₃, see also FIG. 4). The terminal groups are notcritical and may generally be alkyl or substituted alkyl.

Alternatively, the organosilicon material may have two or more siliconatoms like in (CH₃)₃SiO—(CH₂CH₂O)₃—Si(CH₃)₃, or in disiloxaneCH₃(OCH₂CH₂)₃OCH₂CH₂CH₂—Si(CH₃)₂OSi(CH₃)₂—CH₂CH₂CH₂O(CH₂CH₂O)₃CH₃ (seeFIG. 5), or have a polysiloxane chain structure with side groupscontaining such oligoethyleneoxide moieties (see FIG. 6). The length ofthe polysiloxane backbone is not critical.

Examples of other suitable ion-conducting organosilicon materials whichcan be used in embodiments of the present invention include:

Me₃SiOSiMe₂CH₂CH₂CH₂O(CH₂CH₂O)₃Me

Me₃SiOSiMe₂O(CH₂CH₂O)₃Me

Me₃SiOSiMe₂CH₂CH₂CH₂O(CH₂CH₂O)₃SiMe₃

Me₃SiOSiMe₂CH₂O(CH₂CH₂O)₃SiMe₃

Me₃SiOSiMe₂CH₂O(CH₂CH₂O)₃CH₂SiMe₃

Me₃SiOSiMe₂CH₂O(CH₂CH₂O)₃CH₂SiMe₃

Me(CH₂CH₂O)₃OSiMe₂O(CH₂CH₂O)₃Me

Me(CH₂CH₂O)₃O(CH₂CH₂)₃SiMe₂O(CH₂CH₂O)₃Me

Me(CH₂CH₂O)₃O(CH₂CH₂)₃SiMe₂(CH₂CH₂)₃O(CH₂CH₂O)₃Me

Preferred organosilicon electrolytes can be produced as liquids havinghigh room temperature ionic conductivity (σ_(RT)˜1×10⁻³ S/cm). Further,they have compatibility with electrode materials, thermal andelectrochemical stability, the capability of withstanding voltageshigher than for any other known organic or aqueous electrolyte, very lowvolatility, and low flammability and toxicity. The preferredorganosilicon liquid electrolytes produce no toxic vapors uponcombustion or decomposition.

When our organosilicon electrolytes are produced as solids they haveexcellent mechanical stability, applicability to process and greatpackaging versatility. Solid polysiloxane materials are particularlywell suited for this purpose. While they have somewhat lower ionicconductivity, their thermal and chemical stability is higher, and theycan be in solid form at room temperature and are especially useful inall-solid-state supercapacitor fabrication.

Synthesis of various ion-conducting oligosiloxanes and their use ascomponents of electrolyte materials for lithium batteries has beenreported. See Richard Hooper et al., Highly Conductive SiloxanePolymers, 34 Macromolecules 931-936 (2001); R. Hooper et al., Novelsiloxane polymers as polymer electrolytes for high energy densitylithium batteries, 1 Silicon Chemistry 121-126 (2002); and R. Hooper etal., A Highly Conductive Solid-State Polymer Electrolyte Based on aDouble-Comb Polysiloxane Polymer with oligo(ethylene oxide) Side Chains,18 Organometallics 3249-3251 (1999). See also U.S. Pat. 6,337,383, andU.S. patent application publications 2003/0180625 and 2003/0198869. Seegenerally U.S. patent application publications 2004/0197665,2004/0214090 and 2004/0248014.

Once one has the organosilicon ion-conducting material, one also needsto add an appropriate salt (preferably lithium salt) to produceorganosilicon electrolyte material. Lithium salts that have been used inlithium batteries have been successfully tested in formulatingorganosilicon electrolytes. The best results were obtained forlithium-bis-oxalatoborate (LiB(C₂O₄)₂, LiBOB),lithium-tetrafluoroborate, and lithium-(trifluoromethylsulfonyl)-imide(LiN(SO₂CF₃)₂, LiTFSi), but the invention is not restricted to justthese salts.

C. Separator

Separators for use in the EDLCs of the present invention can be ofconventional structure. For example, they can be made of polymer film ofporous structure such as PE, PP, or PTFE films, or other known materialswhich have been used as a separator in an EDLC. See generally separatorfilm products of Celgard Inc., W. L. Gore & Associates Inc., AMTEKResearch International LLC, etc.

Alternatively, solid cross-linked polysiloxane polymers such as thosedescribed below (e.g. in Example 1) could be used as a separator filmwith about 25 to 50 μm thickness, for example.

D. Current Collector

On the exterior surface of the electrode/separator “sandwich” weposition current collectors. These are electro-conductive metal platesor films, like aluminum, nickel, copper, molybdenum, titanium, steel, orany other known electro-conductive material which can be used as acurrent collector in supercapacitors.

EXAMPLE 1

In this example an EDLC device was fabricated from activated carbonnanotubes deposited on stainless steel current collectors, using solidcross-linked organosiloxane polymer as the electrolyte, separator and asa binder.

Fabrication of the device began with preparation of theelectrode/current collector assembly plates from activated carbonnanotubes powder. The multi-walled carbon nanotubes (MWCNTs) used forthis study were produced by chemical vapor deposition and treated withhydrochloric acid at room temperature to extract catalyst and otherimpurities. The nanotubes powder then was washed with distilled waterand dried.

Purified MWCNTs were refluxed with concentrated boiling nitric acid forabout 10 hours for activation, and then washed with distilled waterfollowed by rinsing with ethanol and drying in vacuum 10⁻³ mmHg at 60°C. Activated nanotubes then were dispersed by ultrasound sonication for30 min in dimethylformamide (DMF) to prepare a colloidal suspension.

The colloidal suspension was deposited directly onto two polishedstainless steel disks (8 mm diameter) and dried in vacuum 10⁻³ mmHg at150° C. to produce an assembly of electrode/current collector plates.The total weight of MWCNTs in both plates was 0.9 mg as it was measuredusing micro-analytical balances.

The MWCNTs of the electrodes assembly were then impregnated with across-linkable organosilicon ion-conducting polymer precursor, thecross-linker, the inhibitor and the catalyst dissolved in THF.

The cross-linkable organosilicon polymer precursor was synthesizedaccording to the procedures described in U.S. Pat. No. 6,887,619 (seeFIG. 2). See also the teachings of Z. Zhang et al., Ion conductivecharacteristics of cross-linked network polysiloxane-based solid polymerelectrolytes, 170 Solid State Ionics 233-238 (2004); Z. Zhang et al.,Cross-Linked Network Polymer Electrolytes Based on a PolysiloxaneBackbone with Oligo(oxyethylene) Side Chains: Synthesis andConductivity, 36 J. Macromolecules 9176-9180 (2003); and Z. Zhengchenget al., Network-Type Ionic Conductors Based onOligoethyleneoxy-Functionalized Pentamethylcyclopenta-siloxanes, 38Macromolecules 5714-5720 (2005).

Per FIGS. 2 & 3, and with n=20, p=10, and q=13, the reaction solution,which was prepared by dissolving 85.7 mg of the precursor, 7.8 mg of thecross-linker, 20 mg of lithium salt (LiTFSi) and 5 μL of the Pt-catalystsolution (platinum-divinyltetramethyl-disiloxane, ˜100 ppm, Karlstedt'scatalyst xylene solution, Aldrich) in 1 ml of dry THF, was applied dropby drop to the surface of MWCNTs of the two electrode assemblies until avisible meniscus was formed.

Both impregnated electrodes assembled on current collector plates werethen heated at 80° C. in vacuum for 15 minutes until a gel layer wasformed on the surface of carbon nanotubes and then they were joinedtogether by the gel-polymer layer sides without using any pressure, andthen heated at 80° C. in vacuum for 12 hours. The sandwich structurewith the collector plates on the outside and inner layers of MWCNTsimpregnated with a solid cross-linked organosilicon electrolyte formingthe electrodes, and the cross-linked solid organosilicon electrolyteforming a separator in the inside was produced.

The assembled sandwich device was placed into the Teflon Swagelokelectrochemical cell of FIG. 7, and tested by the impedancespectroscopy, cyclic voltammetry, and galvanostatic charge/dischargemethods. Various tests were conducted on this cell confirming itsability to operate across a wide range of voltages, as well as itsability to operate stably.

EXAMPLE 2

An electric double layer capacitor was prepared in the same manner as inExample 1, except that vertically aligned carbon nanofibers were used asa high surface area carbonaceous component of the electrode material(for two electrodes) instead of activated carbon nanotubes. We used a0.8M solution of LiBOB in liquid disiloxane of FIG. 5 as theelectrolyte, and used a PE/PP/PE tri-layer film (Celgard 3501, 25 μm) asa conventional separator.

Vertically aligned carbon nanofibers were grown on titanium/nickelcoated stainless steel substrates using DC plasma-enhanced chemicalvapor deposition. Typical growth conditions used flow rates of 80standard cubic centimeters per minute (sccm) ammonia and 30 sccmacetylene, with a chamber pressure of 4 Torr and a DC power of 360watts. Each metal layer in Ti/Ni coating had 50 nm thickness.

A two-electrode assembly, containing arrayed nanofiber electrodes ofthis example, was then placed into the Teflon Swagelok electrochemicalcell (FIG. 7) and tested. Test results of the EDLC of this example arepresented in FIGS. 8 & 9.

EXAMPLE 3

Attention is called to FIG. 10 where there is shown an alternativemonolithic nanofiber based EDLC. There is shown a substrate 30preferably made of silicon nitride. The substrate is then coated with athin layer of molybdenum 31, which is in turn coated with a thin layerof titanium 32. Spaced strips of nickel 33/34 are then positioned alongthe structure.

Vertically aligned carbon nanofibers 35 were then grown on the nickellines using DC plasma-enhanced chemical vapor deposition. The growthconditions we used were the same as in Example 2.

In this example, our most preferred form nanofibers were grown onsilicon nitride substrates that we covered with a thin film consistingof 50 nm Mo, followed by 20 nm Ti, and finally 20 nm Ni as the top layerstrips. SEM images (see FIG. 11) show that the nanofibers werevertically aligned with an average diameter of ˜75 nm. The length of thenanofibers was controlled by varying the duration of the growth.Nanofibers used for our tests were obtained with a growth time of 15minutes, which yielded fibers with an average length of approximately 3μm.

One row of fibers over the line 33 formed a first electrode, while asecond row of fibers over the line 34 formed a second electrode, withthe nickel lines 33 and 34 functioning as current collectors. Theresulting structure was encased by a case 37 into which a common pool ofelectrolyte (not shown) functioned as the electrolyte for bothelectrodes.

The lines 33 and 34 should preferably be very close together to maximizethe efficiency of the EDLC. In this regard, lines a few microns aparthave been tested, with preferred spacing in the range of 0.1 micron to1,000 microns.

It should be appreciated that this “monolithic” structure avoids theneed for a separator between the electrodes. The two electrodes can havetheir high surface area “porous” material formed simultaneously.

EXAMPLE 4

In this example the high surface area porous materials were preparedfrom activated carbon (rather than nanotubes or nanofibers), theseparator was a standard PE/PP/PE tri-layer separator (Celgard), anddifferent liquid organosilicon electrolytes were used to fabricate thedevice.

The 8 mg of high purity activated carbon powder with a specific surfacearea of 2000 m2/g and an average particle size of 2 μm (80 wt. %), 1 mgof acetylene black (10 wt. %) and 1 mg poly-tetrafluoroethylene (PTFE)powder (10 wt. %) were mixed in 1 ml of ethanol by ultrasonication toproduce slurry. The slurry then was deposited on one side of an aluminumdisk of 12 mm diameter and 500 μm thickness, with a surface roughenedprior to its use by chemical etching.

After drying in the air for 30 minutes, the coated disk was furtherdried in vacuum (10⁻³ Torr) at 130° C. for 60 minutes to provide aporous carbon electrode material. A pair of the obtained electrodes wascombined together interlaying a standard prior art high molecular weightseparator membrane of 25 μm thickness (Celgard) there between.

The electrode-separator-electrode body was then impregnated by thevacuum soak method with 1 ml of a solution of liquid organosiliconelectrolyte prepared by dissolving LiBOB andpoly(siloxane-g-oligoethyleneoxide) in THF. Quantities of materials wereused to produce 0.5 M concentration of the salt in the polymer. Seegenerally U.S. patent application publication 2003/0180625 for synthesisof these types of polymers.

Thereafter, the impregnated unit was dried in vacuum at 10⁻³ Torr at130° C. and then housed in a Swagelok electrochemical cell with aluminumlead wires and Teflon packing. Varied tests were conducted on this cellconfirming its ability to operate across a wide range of voltages, aswell as its ability to operate stably.

EXAMPLE 5

For this EDLC the Example 1 was followed except thatmethoxyethoxyethoxyethoxytrimethylsilane was used as the ion-conductingorganosilicon component of a liquid organosilicon electrolyte containing0.8 M LiBOB salt to impregnate the MWCNTs before applying the solidcross-linked organosilicon polymer/LiBOB separator layer.

EXAMPLE 6

For this EDLC the device was prepared in the same manner as in Example4, except that solid cross-linked organosilicon electrolyte described inU.S. Pat. No. 6,887,619 was used to fabricate a separator film.

EXAMPLE 7

For this EDLC the device was prepared in the same manner as in Example1, except that the lithium salt LiBOB was used as a component of thesolid organosilicon electrolyte.

EXAMPLE 8

For this EDLC the device was prepared in the same manner as in Example2, except that a solid cross-linked organosilicon electrolyte was usedto impregnate carbon nanofibers and as a separator film.

The assembled sandwich device was placed into the Teflon Swagelokelectrochemical cell of FIG. 7, and tested by the impedancespectroscopy, cyclic voltammetry, galvanostatic charge dischargemethods, confirming its ability to operate across a wide range ofvoltages, as well as its ability to operate stably.

EXAMPLE 9

A similar device could be constructed using the FIG. 5 electrolyte andthe above techniques.

EXAMPLE 10

A similar device could be constructed using the FIG. 6 electrolyte andthe above techniques.

TEST RESULTS

Our testing confirmed that we could construct EDLC devices which couldstably and efficiently operate at a wide variety of voltages, and coulddo so even at voltages higher than 2.7 V, which is the state of the artvalue today for industrial organic electrolytes. See FIG. 9.

OTHER EMBODIMENTS

While a number of embodiments of the present invention have beendescribed above, the present invention is not limited to just thesedisclosed examples. There are other modifications that are meant to bewithin the scope of the invention and claims. For example, themonolithically constructed carbon nanofiber electrodes could be usedwith other liquid electrolytes (regardless of whether organosilicon innature).

Thus, the claims should be looked to in order to judge the full scope ofthe invention.

INDUSTRIAL APPLICABILITY

The present invention provides improved supercapacitors, and improvedelectrodes and separators for use therewith. They could, for example, beused in electric and hybrid-electric vehicles, satellites, windgenerators, photovoltaics, copy machines, high power electronichousehold appliances, electric tools, electric power generation, andelectric distribution systems.

1. An electrochemical double-layer capacitor, comprising: a firstelectrode having a first porous material mixed with a firstorganosilicon electrolyte; and a second electrode; wherein the firstorganosilicon electrolyte is an oligoethyleneoxide substitutedorganosilane electrolyte.
 2. An electrochemical double-layer capacitor,comprising: a first electrode having a first porous material mixed witha first organosilicon electrolyte; and a second electrode, wherein thesecond electrode comprises a second porous material which is the same ordifferent from the first porous material and is mixed with a secondorganosilicon electrolyte which is the same or different from the firstorganosilicon electrolyte.
 3. The electrochemical double-layer capacitorof claim 2, further comprising a separator disposed between the firstand second electrodes.
 4. The electrochemical double-layer capacitor ofclaim 3, wherein the electrode separator also comprises an organosiliconpolymer.
 5. The electrochemical double-layer capacitor of claim 2,wherein the first organosilicon electrolyte is a polysiloxaneelectrolyte.
 6. The electrochemical double-layer capacitor of claim 2,wherein the first and second porous materials are selected from thegroup consisting of porous carbon, non-porous carbon having crystallitessuitable for ions intercalation, porous metal oxides, and carbonmodified with an electroactive polymer, and the first porous material isthe same or different from the second porous material.
 7. Theelectrochemical double-layer capacitor of claim 6, wherein the first andsecond porous materials are selected from the group consisting of porouscarbon nanotubes and carbon nanofibers.
 8. The electrochemicaldouble-layer capacitor of claim 7, wherein the first porous material isa patterned carbon nanofiber material, and the first and secondelectrodes are not separated by a separator.
 9. The electrochemicaldouble-layer capacitor of claim 2, further comprising a first currentcollector adjacent the first electrode and a second current collectoradjacent the second electrode.
 10. The electrochemical double-layercapacitor of claim 9, wherein the first and second current collectorscomprise aluminum.
 11. The electrochemical double-layer capacitor ofclaim 2, wherein the electrolyte of both the first and second electrodefurther comprise a lithium salt.
 12. The electrochemical double-layercapacitor of claim 11, wherein the lithium salt is the same lithium saltfor both the first and second electrode.
 13. The electrochemicaldouble-layer capacitor of claim 12, where in the lithium salt isselected from the group consisting of lithium-tetrafluoro-borate,lithium-bis (trifluoromethylsulfonyl)imide, and lithiumbis-oxalatoborate.
 14. An electrochemical double-layer capacitorcomprising a first electrode and a second electrode, these electrodesboth being formed as part of a patterned array of spaced carbonnanofibers, wherein the electrochemical double layer capacitor does nothave a separator between the electrodes.
 15. The electrochemicaldouble-layer capacitor of claim 14, wherein at least the first electrodefurther comprises an organosilicon electrolyte.